Engineering heterogeneous catalysts with molecular precision is an ongoing challenge to access materials with predictable properties that can be optimized using quantitative structure–activity ...relationships. One approach is grafting organometallic complexes on dehydroxylated oxide supports using surface organometallic chemistry (SOMC). Although fruitful, this technique is limited to complexes containing a σ-bound surface oxygen in the first coordination sphere of the metal. In this perspective, we describe our recent efforts to incorporate molecular diversity onto surfaces to obtain molecularly defined heterogeneous catalysts containing N-heterocyclic carbene ligands for C–H activation, olefin metathesis, CO2 hydrogenation, and the Z-selective semihydrogenation of alkynes.
This work describes the development of easy-to-prepare cobalt nanoparticles (NPs) in solution as promising alternative catalysts for alkene hydrosilylation with the industrially relevant tertiary ...silane 1,1,1,3,5,5,5-heptamethyltrisiloxane (MDHM). The Co NPs demonstrated high activity when used at 30 °C for 3.5–7 h in toluene, with catalyst loadings 0.05–0.2 mol %, without additives. Under these mild conditions, a set of terminal alkenes were found to react with MDHM, yielding exclusively the anti-Markovnikov product in up to 99% yields. Additionally, we demonstrated the possibility of using UV irradiation to further activate these cobalt NPs not only to enhance their catalytic performances but also to promote tandem isomerization–hydrosilylation reactions using internal alkenes, among them unsaturated fatty ester (methyl oleate), to produce linear products in up to quantitative yields.
A novel heterobimetallic tantalum/iridium hydrido complex, {Ta(CH2 t Bu)3}{IrH2(Cp*)} 1, featuring a very short metal–metal bond, has been isolated through an original alkane elimination route from ...Ta(CH t Bu)(CH2 t Bu)3 and Cp*IrH4. This molecular precursor has been used to synthesize well-defined silica-supported low-coordinate heterobimetallic hydrido species SiOTa(CH2 t Bu)2{IrH2(Cp*)}, 5, and SiOTa(CH2 t Bu)H{IrH2(Cp*)}, 6, using a surface organometallic chemistry (SOMC) approach. The SOMC methodology prevents undesired dimerization as encountered in solution and leading to a tetranuclear species {Ta(CH2 t Bu)2}(Cp*IrH)2, 4. This approach therefore allows access to unique low-coordinate species not attainable in solution. These original supported Ta/Ir species exhibit drastically enhanced catalytic performances in H/D exchange reactions with respect to (i) monometallic analogues as well as (ii) homogeneous systems. In particular, material 6 promotes the H/D exchange between fluorobenzene and C6D6 or D2 as deuterium sources with excellent productivity (TON up to 1422; TOF up to 23.3 h–1) under mild conditions (25 °C, sub-atmospheric D2 pressure) without any additives.
A salt metathesis synthetic strategy is used to access rare tantalum/coinage metal (Cu, Ag, Au) heterobimetallic complexes. Specifically, complex Li(THF)2Ta(C t Bu)(CH2 t Bu)3, 1, reacts with ...(IPr)MCl (M = Cu, Ag, Au, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) to afford the alkylidyne-bridged species Ta(CH2 t Bu)3(μ-C t Bu)M(IPr) 2-M. Interestingly, π-bonding of group 11 metals to the TaC moiety promotes a rare alkylidyne alkyl to bis-alkylidene tautomerism, in which compounds 2-M are in equilibrium with Ta(CH t Bu)(CH2 t Bu)2(μ-CH t Bu)M(IPr) 3-M. This equilibrium was studied in detail using NMR spectroscopy and computational studies. This reveals that the equilibrium position is strongly dependent on the nature of the coinage metal going down the group 11 triad, thus offering a new valuable avenue for controlling this phenomenon. Furthermore, we show that these uncommon bimetallic couples could open attractive opportunities for synergistic reactivity. We notably report an uncommon deoxygenative carbyne transfer to CO2 resulting in rare examples of coinage metal ketenyl species, ( t BuCCO)M(IPr), 4-M (M = Cu, Ag, Au). In the case of the Ta/Li analogue 1, the bis(alkylidene) tautomer is not detected, and the reaction with CO2 does not cleanly yield ketenyl species, which highlights the pivotal role played by the coinage metal partner in controlling these unconventional reactions.
Cp*IrH4 reacts with Hf(Np)4 (Np = CH2C(CH3)3) to yield the heterobimetallic complex Hf(Np)3(μ-H)3IrCp*, 2. Treatment of 2 with neopentyl lithium (NpLi) results in the nucleophilic addition of a ...neopentyl moiety onto the Hf center, restoring Hf(Np)4 and displacing the Cp*IrH3 – iridate fragment. By contrast, treatment of the tantalum/iridium analogue Ta(Np)3IrH2(Cp*), 1, with NpLi triggers the α-hydrogen abstraction at a Ta–neopentyl moiety and leads to the formation of the alkylidene -ate complex Li{Ta(CH t Bu)(Np)2}{IrH2Cp*}, 4. The benzylidene analogue K{Ta(CHPh)(Np)2}{IrH2Cp*}, 5, is obtained upon reacting 1 with benzyl potassium. An unusual vinyl oxo species, {Li(THF)3}Ta(O)(Np)2{μ-C2H3}IrH(Cp*), 6, is formed when the reaction of 1 with NpLi is carried out in tetrahydrofuran (THF) due to THF fragmentation.
We report an original alkane elimination approach, entailing the protonolysis of triisobutylaluminum by the acidic hydrides from Cp*IrH4. This strategy allows access to a series of well-defined tri- ...and tetranuclear iridium aluminum polyhydride clusters, depending on the stoichiometry: Cp*IrH3Al(iBu)22 (1), Cp*IrH2Al(iBu)2 (2), (Cp*IrH3)2Al(iBu) (3), and (Cp*IrH3)3Al (4). Contrary to most transition-metal aluminohydride complexes, which can be considered as AlH x+3x– aluminates and LnM+ moieties, the situation here is reversed: These complexes have original structures that are best described as Cp*IrH x n− iridate units surrounding cationic Al(III) fragments. This is corroborated by reactivity studies, which show that the hydrides are always retained at the iridium sites and that the Cp*IrH3− moieties are labile and can be transmetalated to yield potassium (KIrCp*H3, 8) or silver ((AgIrCp*H3 n , 10) derivatives of potential synthetic interest. DFT calculations show that the bonding situation can vary in these systems, from 3-center 2-electron hydride-bridged Lewis adducts of the form Ir–H⇀Al to direct polarized metal–metal interaction from donation of d-electrons of Ir to the Al metal, and both types of interactions take place to some extent in each of these clusters.
Secondary interactions are demonstrated to direct the stability of well-defined Ru–NHC-based heterogeneous alkene metathesis catalysts. By providing key stabilization of the active sites, higher ...catalytic performance is achieved. Specifically, they can be described as interactions between the metal center (active site) and the surface functionality of the support, and they have been detected by surface-enhanced 1H–29Si NMR spectroscopy of the ligand and 31P solid-state NMR of the catalyst precursor. They are present only when the metal center is attached to the surface via a flexible linker (a propyl group), which allows the active site to either react with the substrate or relax, reversibly, to the surface, thus providing stability. In contrast, the use of a rigid linker (here mesitylphenyl) leads to a well-defined active site far away from the surface, stabilized only by a phosphine ligand which under reaction conditions leaves probably irreversibly, leading to faster decomposition and deactivation of the catalysts.
The structural characterization of supported molecular catalysts is challenging due to the low density of active sites and the presence of several organic/organometallic surface groups resulting from ...the often complex surface chemistry associated with support functionalization. Here, we provide a complete atomic-scale description of all surface sites in an N-heterocyclic carbene based on iridium and supported on silica, at all stages of its synthesis. By combining a suitable isotope labeling strategy with the implementation of multinuclear dipolar recoupling DNP-enhanced NMR experiments, the 3D structure of the Ir-NHC sites, as well as that of the synthesis intermediates were determined. As a significant fraction of parent surface fragments does not react during the multistep synthesis, site-selective experiments were implemented to specifically probe proximities between the organometallic groups and the solid support. The NMR-derived structure of the iridium sites points to a well-defined conformation. By interpreting EXAFS spectroscopy and chemical analysis data augmented by computational studies, the presence of two coordination geometries is demonstrated: Ir-NHC fragments coordinated by a 1,5-cyclooctadiene and one Cl ligand, as well as, more surprisingly, a fragment coordinated by two NHC and two Cl ligands. This study demonstrates a unique methodology to disclose individual surface structures in complex, multisite environments, a long-standing challenge in the field of heterogeneous/supported catalysts, while revealing new, unexpected structural features of metallo-NHC-supported substrates. It also highlights the potentially large diversity of surface sites present in functional materials prepared by surface chemistry, an essential knowledge to design materials with improved performances.
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